The semiconductor devices may be roughly classified into a lateral device, that arranges the main electrodes thereof on one major surface, and a vertical device that distributes the main electrodes thereof on two major surfaces facing opposite to each other. In the vertical device, a drift current flows in the ON-state of the device and depletion layers expand in the OFF-state of the device both in the thickness direction of the substrate thereof (vertically). FIG. 9 is a cross sectional view of a conventional planar-type vertical n-channel MOSFET. Referring now to FIG. 9, the vertical MOSFET includes an n+-type drain layer 11 with low electrical resistance; a drain electrode 18 in electrical contact with the back surface of n+-type drain layer 11; a highly resistive n-type drain drift layer 12 on n+-type drain layer 11; p-type base regions 13 (p-type well regions or channel diffusion regions) formed selectively in the surface portion of drain drift layer 12; a heavily doped n+-type source region 14 formed selectively in the surface portion of p-type base region 13; a heavily doped p+-type contact region 19 formed selectively in the surface portion of p-type base region 13; a gate insulation film 15 on the extended portion of p-type base region 13 extended between n-type drain drift layer 12 and n+-type source region 14; a polysilicon gate electrode layer 16 on gate insulation film 15; and a source electrode 17 in common contact with n+-type source region 14 and p+-type contact region 19.
Highly resistive n-type drain drift layer 12 provides a vertical drift current path in the ON-state of the MOSFET and is depleted in the OFF-state of the MOSFET to increase the breakdown voltage. Shortening the current path in n-type drain drift layer 12 is effective to substantially reduce the on-resistance (the resistance between the source and the drain) of the MOSFET, since the drift resistance is reduced. However, the shortening the current path in n-type drain drift layer 12 narrows the expansion width of the depletion layer expanding from the pn-junction between p-type base region 13 and n-type drain drift layer 12. Since the depletion electric field strength soon reaches the maximum electric field (critical electric field) of silicon due to the narrowed expansion width of the depletion layer, the breakdown voltage (the voltage between the drain and the source) of the MOSFET is reduced. Although a high breakdown voltage is obtained by thickening n-type drain drift layer 12, thick n-type drain drift layer 12 inevitably causes on-resistance increase, that further causes on-loss increase. In other words, there exists a tradeoff relation between the on-resistance (current capacity) and the breakdown voltage. The tradeoff relation between the on-resistance and the breakdown voltage exists also in the other semiconductor devices such as IGBT's, bipolar transistors and diodes.
European Patent 0 053 854, U.S. Pat. No. 5,216,275, U.S. Pat. No. 5,438,215, Japanese Unexamined Laid Open Patent Application H09-266311 and Japanese Unexamined Laid Open Patent Application H10-223896 disclose semiconductor devices, which include an alternating conductivity type layer formed of heavily doped n-type regions and heavily doped p-type regions alternately arranged with each other to obviate the problems described above.
FIG. 10 is a cross sectional view of the vertical MOSFET disclosed in U.S. Pat. No. 5,216,275. The vertical MOSFET of FIG. 10 is different from the vertical MOSFET of FIG. 9 in that the vertical MOSFET of FIG. 10 includes a drain drift layer 22, that is not of one conductivity type but of alternating conductivity types and formed of n-type drift current path regions 22a and p-type partition regions 22b alternately arranged with each other. Even if the impurity concentrations in the alternating conductivity type layer are high, a high breakdown voltage will be obtained, since depletion layers expand laterally, in the OFF-state of the device, from multiple pn-junctions extending vertically across the alternating conductivity type layer.
Drain drift layer 22 is formed in the following way. An n-type layer is grown epitaxially on an n+-type drain layer 11 as a substrate. Trenches are dug through n-type layer down to n+-type drain layer 11, leaving n-type drift current path regions 22a. Then, p-type partition regions 22b are epitaxially grown selectively in the trenches. Hereinafter, the semiconductor device including a drain drift layer of alternating conductivity types as described above will be referred to sometimes as the “super-junction semiconductor device”.
Detailed dimensions of the super-junction semiconductor device disclosed in U.S. Pat. No. 5,216,275 are as follows. The thickness of drain drift layer 22 is described with a breakdown voltage VB by 0.024VB1.2 (μm). When the thickness of n-type drift current path regions 22a and the thickness of p-type partition region 22b are the same b and the impurity concentrations in n-type drift current path regions 22a and p-type partition region 22b are the same N, the impurity concentration and the thickness b are related with each other by N=7.2×1016VB0.2b (cm3). When VB is 800 V and b is 5 μm, the impurity concentration N is 1.9×1016 cm3. Since the impurity concentration in the conventional drain drift layer of one conductivity type is around 2×1014 cm3, the drain drift layer of alternating conductivity types facilitates realizing a high impurity concentration therein, reducing the on-resistance and providing the semiconductor device with a high breakdown voltage.
However, the trenches for forming p-type partition regions 22b are narrow and deep. It is difficult for the presently available selective etching techniques to dig the trenches with such a large aspect ratio, and it is difficult for the presently available epitaxial growth techniques to grow a high-quality single crystal layer in such a narrow and deep trench. Since it is required to further narrow and thicken each region in the drain drift layer of alternating conductivity types to obtain a higher breakdown voltage, the aspect ratio of the trenches for forming the p-type partition regions should inevitably be larger. Obviously, the use of trenches for forming the p-type partition regions causes a limit for obtaining a higher breakdown voltage and, therefore, is not so practical.
In view of the foregoing, it is an object of the invention to provide a semiconductor device including an improved drain drift layer structure of alternating conductivity types, that is easy to manufacture. It is another object of the invention to provide a semiconductor device that facilitates realizing a high current capacity and a high breakdown voltage. It is still another object of the invention to provide a method of manufacturing the semiconductor device.